fix(autocomplete): panel blending in with background in high contrast mode (#11190)

Fixes the autocomplete panel not being visible in high contrast mode, because it doesn't have a border and box shadows aren't being rendered.

src/lib/autocomplete/autocomplete.scss

@@ -1,4 +1,5 @@
 @import '../core/style/menu-common';
+@import '../../cdk/a11y/a11y';
 
 /**
  * The max-height of the panel, currently matching mat-select value.
@@ -22,4 +23,8 @@ $mat-autocomplete-panel-max-height: 256px !default;
   &.mat-autocomplete-hidden {
     visibility: hidden;
   }
+
+  @include cdk-high-contrast {
+    outline: solid 1px;
+  }
 }